Fig. 4.5C
Satellite cells encapsulate dorsal root and cranial nerve ganglion cells and regulate their microenvironment in a fashion similar to that of astrocytes.
Microglia are derived from erythromyeloid stem cells that migrate into the CNS early in development. They play an important role in immune responses within the CNS. When the CNS is damaged, microglia help remove the cellular products of the damage by phagocytosis. They are assisted by other glia and by other phagocytes that invade the CNS from the circulation. In addition to their role in immune responses, recent evidence suggests they are also active in healthy brain tissue and may have important roles in normal brain development and function, including pruning of excess synapses that are formed during development and synaptic plasticity.
Ependymal cells form the epithelium lining the ventricular spaces of the brain, which contain (cerebrospinal fluid) CSF. CSF is secreted in large part by specialized ependymal cells of the choroid plexuses located in the ventricular system. Many substances diffuse readily across the ependyma, which lies between the extracellular space of the brain and the CSF.
Mostneuronsintheadultnervoussystemarepostmitoticcells(althoughsomestemcellsmayalsoremainincertainsitesinthebrain).Manyglialprecursorcellsarepresentintheadultbrain,andtheycanstilldivideanddifferentiate.Thusthecellularelementsthatgiverisetomostintrinsicbraintumorsintheadultbrainaretheglialcells.Forexample,braintumorscanbederivedfromastrocytes(whichvaryinmalignancyfromtheslowlygrowingastrocytomatotherapidlyfatalglioblastomamultiforme),fromoligodendroglia(oligodendroglioma),orfromependymalcells(ependymoma).Meningealcellscanalsogiverisetoslowlygrowingtumors(meningiomas)thatcompressbraintissue,ascanSchwanncells(e.g.,acousticschwannomas,whicharetumorsformedbySchwanncellsoftheeighthcranialnerve).Inthebrainofinfants,neuronsthatarestilldividingcansometimesgiverisetoneuroblastomas(e.g.,oftheroofofthefourthventricle)orretinoblastomas(intheeye).
The Peripheral Nervous System
The PNS provides an interface between the environment and the CNS, both for sensory information flowing to the CNS and for motor commands issued from the CNS. It includes sensory (or primary afferent) neurons, somatic motor neurons, and autonomic motor neurons.
Sensory pathways into the nervous system start with a receptor, which may simply be a specialized part of an axon in the PNS or may include additional cells. Each sensory receptor is organized so that it transduces a specific type of energy into an electrical signal. Thus they can be classified in terms of the type of energy they transduce (e.g., photoreceptors transduce light, mechanoreceptors transduce displacement and force). They may also be classified according to the source of the input (e.g., exteroceptors signal external events, proprioceptors signal the state of a body part such as the angle of elbow, and interoceptors signal the distension of the gut).
The transduction process leads to an electrical response in the primary afferent called a receptor potential, which triggers action potentials in the primary afferent fibers innervating the receptor. These action potentials contain information about the sensory stimulus that is conveyed to the CNS via the primary afferent.